1. Field of the Invention
The present invention relates to circuitry for providing voltage surge suppression to protect electronic equipment from voltage surges occurring through a common household outlet, through a coaxial cable, or through a telephone line coupled to the electronic equipment.
2. Description of the Related Art
It is desirable to provide voltage surge suppressors to protect electronic equipment from transient voltage surges. Various types of household electronic products can be damaged by short duration transient voltage surges which are supplied to the products from a common household outlet or through electrical wiring such as a coaxial cable.
There are several known methods for providing transient voltage surge suppression for electronic products. One known method for protecting low voltage signal circuitry is shown in FIG. 1. A circuit 10 is provided for use with a coaxial, cable 11 including a signal conductor 12 and a shield conductor 14. The circuit 10 includes a gas tube 16 coupled across the signal conductor 12 and the shield conductor 14. The shield conductor 14 is coupled directly to ground. If the voltage across signal conductor 12 exceeds a predetermined level, the gas in tube 16 is ionized, permitting current to flow between the two electrodes inside gas tube 16. Therefore, when a voltage surge across signal conductor 12 exceeds the level at which the ionized gas current pathway forms, the voltage is clamped at the ionization voltage of gas tube 16.
Another known surge suppression circuit for a coaxial cable 11 is illustrated in FIG. 2. The circuit 18 includes a gas tube 20 coupled across the signal conductor 12 and shield conductor 14 of coaxial cable 11. A varistor 22, such as a metal oxide varistor (hereinafter "MOV"), is coupled between shield conductor 14 and ground. The surge protection circuit 18 illustrated in FIG. 2 is disclosed in U.S. Pat. No. 4,903,161. In circuit 18, if a voltage surge is generated on signal conductor 12, the voltage surge is first clamped by gas tube 20 to shield conductor 14 and then by MOV 22 to ground. If a voltage surge is generated on shield conductor 14, the voltage surge is clamped through MOV 22 to ground. The MOV 22 in circuit 18 eliminates the "ground rise" effect which occurs if a voltage surge comes from the ground line. Therefore, circuit 18 illustrated in FIG. 2 is an improvement over circuit 10 illustrated in FIG. 1. However, by adding MOV 22 between the shield conductor 14 and ground, the voltage level required to clamp a voltage surge occurring across signal conductor 12 to ground is increased by an amount equal to the MOV 22 clamping voltage rating. In other words, a larger voltage surge across signal conductor 12 is required before the initiation of voltage surge protection in circuit 18 as compared with circuit 10.
FIG. 3 illustrates two additional types of known surge protection circuitry. Circuit 24 includes a conventional voltage surge suppression circuit used with a household outlet having a line conductor 26, a neutral conductor 28, and a ground conductor 30. Voltage surges can be generated from line 26 to neutral 28, line 26 to ground 30, or neutral 28 to ground 30. The circuit 24 includes a first MOV 32 coupled across the line conductor 26 and the neutral conductor 28, a second MOV 33 coupled across the line conductor 26 and the ground conductor 30, and a third MOV 34 coupled across the neutral conductor 28 and ground conductor 30.
Circuit 25 is a voltage surge suppression circuit typically used with low voltage signal data line connections including conductors 36 and 38 illustrated in FIG. 3. Gas tubes 40 and 42 are used to couple conductors 36 and 38, respectively, to ground 30.